The invention relates to composite articles wherein a solid glass body is adhered to a supporting network structure or core. The adherence may be through the medium of a fused glass sealing frit.
The invention arose from problems encountered in producing a telescope mirror blank of the type described in my U.S. Pat. No. 4,917,934. The invention finds particular application in production of such an article, and, therefore, is primarily described with reference thereto. However, it will be readily apparent that it is equally applicable to composite articles in general that have similar characteristics and problems.
Telescope mirror blanks are commonly produced from materials having very low coefficients of thermal expansion (CTEs), particularly over the temperature range of -50.degree. to 100.degree. C. This minimizes the effect of temperature changes in the ambient atmosphere on measurements made with a telescope.
Materials presently used include both fused silicas and glass-ceramics having CTE values in the range of .+-.10.times.10.sup.-7 /.degree.C. over the 0.degree.-300.degree. C. range. The fused silicas are an essentially pure silica and a silica doped with about 7.5% TiO.sub.2. The latter is described in detail in U.S. Pat. No. 2,326,059 (Nordberg), and is hereafter identified as a TiO.sub.2 -doped, fused silica glass.
The advent of larger diameter, and hence much heavier, mirror blanks led to producing the face plate and the network core components separately. Subsequently, these components are united, for example, by use of a sealing frit.
My U.S. Pat. No. 4,917,934 describes producing a network core as a separate body. The core is then united with the face and back plates by frit sealing. The network core is assembled from a plurality of sintered struts. The struts are formed from a thermally crystallizable glass by ceramic powder technology and then sintered. The sintered struts are sealed to one another, and to the plates, with a sealing glass frit. Preferably, the same thermally crystallizable glass frit is used for both sealing and for production of the struts.
The thermally crystallizable glasses recommended for forming the network core are described in U.S. Pat. No. 4,315,991 (Hagy et al.). The glasses disclosed there are compatible with materials having coefficients of thermal expansion (CTEs) in the range of -5 to +5.times.10.sup.-7 /.degree.C. (0.degree.-300.degree. C.). The glasses sinter and crystallize in the temperature range of 900.degree.-1000.degree. C.
As expressed in weight percent on an oxide basis, these glasses consist essentially of 1-2% Li.sub.2 O, 0.7-1.5% MgO, 9-13% ZnO, 19-23% Al.sub.2 O.sub.3, 62-68% SiO.sub.2, and at least 1% K.sub.2 O+Rb.sub.2 O+Cs.sub.2 O in the indicated proportions of 0-3% K.sub.2 O, 0-4% Rb.sub.2 O and 0-6% Cs.sub.2 O. The heavy alkali metal oxides inhibit crystallization, and thus facilitate sintering. The primary crystal phase developed during crystallization is zinc beta-quartz.
A production glass was selected to provide as small an expansion mismatch with the face plate as possible in the vicinity of ambient temperature (-50.degree. to +50.degree. C.). At the same time, a reasonably close match had to be maintained at higher temperatures as well. An ambient temperature average mismatch as low as about 0.30 ppm/.degree.C. could be obtained by carefully tailoring the glass composition. This proved satisfactory for frit sealing, but would not suffice for strut production. For that purpose, modeling studies showed that a value not over 0.15 ppm/.degree.C., and preferably not over 0.10 ppm/.degree.C., would be required.